1. Field of the Invention
The present invention relates to a method of forming a binary code of a read-only memory (ROM), and more particularity, to a method of writing a set of binary codes into a ROM.
2. Description of the Prior Art
A read-only memory (ROM) is a semiconductor device that comprises a plurality of memory cells for storing data. Each of the memory cells comprises a MOS transistor. The data held in the circuits of a ROM does not change in either power off or power on conditions. Consequently, the data stored in the ROM will not be lost if the power is turned off. This is also why the ROM can only be read.
The prior art method of forming a ROM involves arranging a plurality of MOS transistors in a matrix format on a predetermined area of a die. These MOS transistors are regarded as the memory cells for storing data. Some of them are enabled to indicate one binary state, whereas others are disabled to indicate the opposite binary state. To arrange these memory cells, a photo mask is formed according to a set of binary codes to be written into the ROM. Then, the pattern of the photo mask is transferred onto the ROM by performing a photolithography process to disable some of the MOS transistors. Therefore, the set of binary codes is written onto the ROM correctly, and the ROM, which is formed from a photo mask, is called Mask ROM.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram of a set of binary codes 10 for introducing a prior art method. FIG. 2 is a schematic diagram of a photo mask 12 formed according to the set of binary codes 10 shown in FIG. 1. The set of binary codes 10 to be written into the ROM is arranged in a matrix format and corresponds to the same cell regions of the photo mask 12. These binary codes are either a 0 or a 1. The cell regions on the photo mask 12 are transparent if their corresponding binary code is 0 and opaque if their corresponding binary code is 1.
The prior art method of writing a set of binary codes 10 into a ROM is to transfer the pattern of the photo mask 12 onto the ROM by performing a photolithographic process. A photo mask 12 is formed by exposure and development processes. As shown in FIG. 2, the photo mask 12 comprises a plurality of cell regions arranged in a matrix format, and each of the cell regions can be either a transparent region 14 or an opaque region 16. These transparent and opaque regions will form patterns with "holes" 18 and/or "islands" 17.
Please refer to FIG. 3 to FIG. 5. FIG. 3 to FIG. 5 are schematic diagrams of a method for writing the set of binary codes 10 into a ROM 22. As shown in FIG. 3, the ROM 22 is on a predetermined area of a die 20. The ROM 20 comprises a plurality of memory cells 24 arranged in a matrix format on the die 20, and each of the memory cells 24 comprises a MOS transistor (not shown). A prior art method for writing the set of binary codes 20 onto the ROM 22 is performed by forming a photo mask 12 according to the set of binary codes 10 to be written onto the ROM 22 and then transferring the pattern of the photo mask 12 onto the ROM by a photolithographic process. The cell regions arranged in a matrix format on the photo mask 12 correspond to the memory cells 24 of the ROM 22.
As shown in FIG. 4, the photolithographic process is performed using the photo mask 12, and a photoresist layer 26 is formed on the ROM 22. The memory cells 24 corresponding to the opaque regions 16 of photo mask 12 are covered by the photoresist layer 26. After that, an ion implantation process is performed which will implant the memory cells 24 not covered with the photoresist layer 26 with dopants. Consequently, an ion implantation area 29 is formed (FIG. 5). After removing the photoresist layer 26, the photolithographic process is complete. The gate threshold voltage of the MOS transistor in the ion implantation area 29 is raised. The MOS transistors in the ion implantation areas 29 thus become depletion transistors and can not be used. This fulfills the design requirements of the ROM 22.
Please refer to FIG. 6. FIG. 6 is a graph showing the ranges of focus and exposure energy restrictions while performing the exposure process using the photo mask with the hole 18 and the island 17. These transparent and opaque regions of the photo mask 12, resulting directly from the set of binary codes 10, will form holes 18 and islands 17. According to prior art results, the process restriction parameter ranges might not be the same for holes 18 and islands 17 while performing an exposure. As shown in FIG. 6, range A is the range of focus and exposure energy restrictions while performing an exposure using a photo mask with a hole (a transparent region); range B is the range of focus and exposure energy restrictions while performing an exposure using a photo mask with an island (an opaque region); range C is the range of focus and exposure energy restrictions while performing an exposure using a photo mask with both holes and islands. Because of the limits on the process, it is not easy to control the focus and the exposure energy to remain in range C. Therefore, optical proximity effects may easily occur while performing the exposure process using a photo mask with both holes and islands. In such cases, the pattern of the photo mask cannot be precisely transferred onto the ROM 22, and thus the set of binary codes 10 will also not be written onto the ROM 22 correctly.
It is very common for the prior art photo mask 12, formed directly according to the set of binary codes 10, to have holes 18 and islands 17. When performing the photolithographic process using a mask with these types of patterns, because of the restriction of process parameters, the information on the mask may not be properly transferred to the ROM MOS matrix, and thus the set of binary codes 10 will be written onto the ROM 22 incorrectly.